Sodium Phosphate
Properties
| State | Solid at room temperature |
| Color | White crystalline or granular powder |
| Solubility | Soluble in water (14.5 g/100 mL at 25 °C) |
| Melting Point | 1583 °C |
About Sodium Phosphate
Trisodium phosphate (Na3PO4, molar mass 163.941 g/mol) is the fully deprotonated sodium salt of phosphoric acid, and that is exactly what makes its 1% aqueous solutions sit at pH 12 — the PO4 3- anion is a strong enough base to pull a proton from water in two stages (PO4 3- + H2O -> HPO4 2- + OH-, then HPO4 2- + H2O -> H2PO4 - + OH-). Painters and contractors know it as TSP, the powdered grease-cutter you mix into a bucket before sanding old paint off woodwork. Its industrial heyday was as the primary builder in laundry detergent powders through the 1960s and 1970s — it sequestered Ca2+ and Mg2+ hardness ions as soluble complexes, raised the wash-water pH to assist surfactant action, and emulsified oily soils. The phosphate ban that rolled across US states between 1971 and 1994 ended that use, after Lake Erie eutrophication studies traced algal blooms directly to laundry phosphate runoff. In biochemistry, the conjugate-acid pair H2PO4 - / HPO4 2- (pKa2 = 7.21) is the buffer of choice for everything that runs near physiological pH: cell-culture media, PCR mixes, enzyme kinetics, SDS-PAGE running buffers. In food processing, the various sodium phosphates (mono, di, tri) function as emulsifiers in processed cheese — they sequester calcium from the casein micelles, preventing the protein from coagulating when the cheese is melted.
Where you'll encounter it
If you have ever stripped a kitchen wall before repainting, the gritty white powder in the box labeled TSP that you dissolved in warm water to scrub down the surface was trisodium phosphate. The grease-cutting works through saponification — the high pH hydrolyzes ester bonds in old cooking oils and skin oils that have soaked into the paint. In a molecular biology lab, the 50 mM sodium phosphate buffer at pH 7.4 you used for a Western blot transfer or a Bradford protein assay was almost certainly mixed from Na2HPO4 and NaH2PO4 in a calculated ratio using the Henderson-Hasselbalch equation. Hospital pharmacies still use sodium phosphate enemas as a rapid colon-cleansing prep before colonoscopy, although they have been largely replaced by polyethylene glycol preparations because the osmotic phosphate load can cause acute phosphate nephropathy in elderly or dehydrated patients.
Common Uses
- Heavy-duty cleaner for paint preparation, mildew removal, and grease cutting on hard surfaces
- Buffer component for sodium phosphate buffer at physiological pH 6.8 to 8.0 in molecular biology
- Emulsifying salt in processed cheese to prevent fat separation during melting
- Water softening builder in industrial detergents and metal-cleaning formulations
- Boiler-water treatment to precipitate calcium and prevent scale on heat-exchange surfaces
- Osmotic laxative and colonoscopy-prep ingredient in clinical sodium phosphate enemas
- Pulp and paper processing as a deinking and pH-control agent
- Alkaline cleaner for stainless steel and aluminum in food-processing equipment
Safety Information
GHS: H315 (skin irritation, Category 2), H319 (serious eye irritation, Category 2A), H335 (respiratory irritation, Category 3). No specific OSHA PEL for TSP itself, but inhalable dust falls under the OSHA particulates not otherwise regulated PEL of 15 mg/m3 total dust and 5 mg/m3 respirable. Concentrated solutions above pH 11.5 cause chemical burns on prolonged skin contact. Acute phosphate nephropathy is a documented risk with oral sodium phosphate purgatives in patients with reduced kidney function. Wear nitrile gloves and splash goggles when mixing solutions above 5%; rinse contaminated skin promptly with water. Compatible with most common construction materials but corrosive to aluminum and zinc at high concentrations.
This safety summary is for educational reference only and may not be complete. It is not a substitute for Safety Data Sheets (SDS), medical advice, or professional chemical safety guidance. Always consult appropriate SDS and qualified professionals before handling chemicals.